To form ceramic honeycombs useful for applications such as catalytic converters and diesel particulate filters (DPFs), ceramic particulate precursors are mixed with organic additives (e.g., binders and lubricants) and a liquid medium, which is typically water to form a plastic material. The plastic material is then extruded to form the honeycomb shape, which is subsequently dried to remove the water. The dried honeycomb is then heated to remove the organic additives. After removal of the organic additives the honeycomb is heated to a higher temperature to fuse the ceramic grains so that the honeycomb has the mechanical integrity and microstructure useful for a catalytic converter or DPF.
The heating to remove the organic additives has typically been done in air or oxygen containing atmospheres. Unfortunately, the organic additives invariably display an exothermic reaction associated with their oxidation, which results often in cracking of the body due to localized thermal gradients.
To avoid such cracking, inert atmospheres or low oxygen containing atmospheres (i.e., less than oxygen than air) have been used (see, for example, U.S. Pat. Nos. 6,099,793 and 6,287,509). Unfortunately, the use of such atmospheres tends to remove the organic additives more slowly and leave behind deleterious carbonaceous residue, which quite often impedes the fusing of the ceramic grains at higher temperature or results in undesirable microstructures such as large pores that act as defects.
Another solution has been to pass air, oxygen containing atmosphere or other atmosphere through the honeycomb to minimize the thermal gradients associated with the oxidation of the organic additives (see, for example, U.S. Pat. No. 4,927,577). This method suffers from expensive complexity and becomes less useful as the honeycombs become longer.
More recent solutions have involved using 2 or more organic binders where one is subsequently extracted using a liquid for one of the binders and the second binder is removed by heating using a known heating method such as one of those just described (see, for example, U.S. Patent Publication No. 2004/0079469). This method again suffers from complexity and the handling of more fragile part during the liquid extraction of one of the binders.
Accordingly, it would be desirable to provide both a formation method and a ceramic material that solves one or more of the problems of the prior art, such as one of those described above.